1. Field of the Invention
The present invention relates to random fiber lasers. More specifically, the present invention is directed to narrow line-width Brillouin random fiber laser (BRFL) and laser characterization technique based on the BRFL.
2. Related Art
Narrow line-width lasers have utility in a wide range of applications, such as optical fiber sensing, satellite and optical fiber communication, high resolution spectroscopy, microwave generation and so on.1,2,3,4 A narrow line-width property allows for greater transport capacity and range which is useful for high data rate coherent optical communication system. Narrow line-width lasers such as single mode (SM) fiber lasers or tunable external cavity lasers (ECL), for example, with line-widths in the range of 1 kHz to sub-MHz, are increasingly used as the pump lasers in optical interferometric and fiber sensing systems such as optical interferometers, Brillouin optical time domain reflectometer (BOTDR), phase-sensitive optical time domain reflectometer (Phase-OTDR) and optical frequency domain reflectometer (OFDR).5,6,7 Reliable characterization of laser line-width is a key requirement for many laser-based applications. However, conventional methods and systems for laser line-width measurement either offer insufficient measurement resolution or require costly operational features which can limit their intended application scope.
The highest spectral resolution of diffraction grating based instruments such as optical wavelength meter or optical spectrum analyzer (OSA) is generally in the range of a few MHz, which is not adequate for characterization of sub-MHz line-width laser.8,9 The spectral resolution of Scanning Fabry-Perot interferometers (FPI) are typically in the order of MHz, or sub-MHz with small a tuning range (˜1 GHz).10 
Commonly used homodyne and self-heterodyne techniques based on Mach-Zehnder interferometers (MZI), often applied to measure the laser line-width, can achieve a high resolution.11,12 However, in order to realize a high resolution (e.g. kHz range), an ultra-long delay requiring hundreds of kilometers of delay fiber may be required in both cases. In addition, an external frequency shifter is generally required in order to shift the detected frequency away from the zero frequency of Electrical Spectrum Analyzer (ESA) in order to avoid the low frequency noise (l/f-noise). To avoid the extremely long delay fiber and its induced high loss, the loss-compensated recirculating delayed self-heterodyne interferometer (LC-RDSHI) has been developed for measuring ultra-narrow line-widths.13,14 In this system the delay fiber does not need to be longer than the laser coherence length, but the multi-interferences from recirculation may affect the measurement accuracy. Heterodyne technique is an effective method for high resolution line-width measurement, however it requires a reference laser source with a narrower line-width than that of the test laser and a center frequency that is close to that of the test laser in order for the resulting beat signal to be within the operating bandwidth of the photo-detector (less than tens of GHz).15 
Recently the heterodyne method based on Brillouin fiber ring laser (BFRL) has been used to achieve high resolution spectral analysis and line-width measurement.16,17 This method involves the detection of the heterodyne signal between the test laser and a BFRL signal resulting in measurement resolution of 300 Hz18. However, its implementation requires two laser sources.19 Accordingly, since the narrow line-width property is of key importance in a multitude of applications, a high resolution line-width characterization technique for ensuring that a laser meets the performance requirements of the particular application for which it is being utilized is highly desirable.